1. Field of the Invention
The present invention relates to a piezoelectric single crystal and an ultrasonic probe which includes a piezoelectric element consisting of the piezoelectric single crystal and is useful as a medical diagnostic apparatus or the like.
2. Description of the Related Art
An ultrasonic probe includes an ultrasonic transmitting/receiving element consisting essentially of a piezoelectric element having a pair of electrodes. The ultrasonic probe emits ultrasonic waves against an object to be inspected and receives echoes reflected by interfaces having different acoustic impedances in the object, thereby forming an image of the internal state of the object. An ultrasonic imaging apparatus incorporating this ultrasonic probe is applied to a medical diagnostic apparatus for inspecting the interior of a human body, or to an inspection apparatus for detecting flaws in a metal welded portion.
Recently, an apparatus has been developed, as one medical diagnostic apparatus, which employs a "color flow mapping (CFM) method" capable of two-dimensionally displaying in color the velocity of a bloodstream in, e.g., a heart, a liver, or a carotid artery, by using a Doppler shift of ultrasonic waves caused by a bloodstream, as well as displaying tomographic images (B mode images) of human bodies. Diagnostic capability has been dramatically improved by this medical diagnostic apparatus. Medical diagnostic apparatuses using this CFM method are currently used in diagnosing all organs of a human body, such as the uterus, the liver, and the spleen, and extensive studies have been made to aim at developing an apparatus capable of diagnosing coronary thrombus.
In the case of the B mode image, it is required that high-resolution images be obtained with a high sensitivity, in order for a small change to a morbid state or a vacant space resulting from a physical change to be clearly seen to the depth. In the Doppler mode by which CFM images can be obtained, echoes reflected by fine blood cells a few micrometers in diameter are used. Therefore, signal levels obtained in this Doppler mode are small compared to those in the B mode, so it is necessary to raise the sensitivity of the Doppler mode.
The above-mentioned ultrasonic probe itself or its peripheral circuit has been variously improved in order to obtain a high sensitivity. For example, when the ultrasonic probe is used in detection for the B mode image, a piezoelectric element of an ultrasonic transmitting/receiving element has a large influence on the result obtained. The piezoelectric element for use in the ultrasonic probe must be made from a material with a large dielectric constant by which a large electromechanical coupling coefficient is attained and matching with a transmitting/receiving circuit can be obtained easily. The matching requirement is imposed in order to decrease a loss resulting from a cable or a stray capacitance. For this reason, the piezoelectric elements are formed primarily from a lead zirconate titanate (PZT) piezoelectric ceramic.
The main stream of ultrasonic probes is an array-type ultrasonic probe in which a few tens to about 200 ultrasonic transmitting/receiving elements each having a strip piezoelectric element are arranged. The number of ultrasonic transmitting/receiving elements tends to increase with increasing demand for a high resolution. However, the diameter of the ultrasonic transmitting/receiving surface of the array-type ultrasonic probe cannot be made large if the probe is to be brought into contact with a living body. Consequently, the impedance of each piezoelectric element increases as the number of ultrasonic transmitting/receiving elements increases. This makes it difficult to obtain matching with a transmitting/receiving circuit.
To solve the above problem, U.S. Pat. No. 4,958,327 has disclosed the use of a piezoelectric element consisting of a material with a large dielectric constant or the use of a layered structure of piezoelectric elements. DE3729731A1, on the other hand, has disclosed the use of an impedance converter. However, the electromechanical coupling coefficient of the PZT-based ceramic described above decreases if its relative dielectric constant exceeds 3000. This introduces a problem of low sensitivity again. In the layered structure, on the other hand, although the transmission sensitivity increases in proportion to the number of stacked layers, the reception sensitivity is inversely proportional to the stacked layer number. Therefore, a field to which the piezoelectric element of this type is applicable is limited to a special application in which, for example, the piezoelectric element is smaller than an ordinary one or the cable is long. Also, the use of an impedance converter such as an emitter follower leads to an increase in size of an ultrasonic probe and to band narrowing which is brought about by the specific frequency characteristics inherent in the impedance converter.
Other well-known examples of the piezoelectric material of the piezoelectric element are a single crystal such as lithium niobate, a ceramic such as lead titanate and lead metaniobate, and a polymeric material such as polyvinylidene fluoride or its copolymer. However, piezoelectric elements consisting of these materials are impractical because of their small dielectric constants and electromechanical coupling coefficients. A composite piezoelectric element, such as 1-3 type in which a columnar piezoelectric ceramic is embedded in a resin, is also known. Since, however, the dielectric constant of this piezoelectric element is also small, the element is unsuitable for an array-type ultrasonic probe in which a plurality of strip piezoelectric elements are arranged.
Kuwata et al. have reported in "Japan J. Appl. Phys. 21 (1982)" that a rod piezoelectric element consisting of a solid-solution single crystal of lead zinc niobate and lead titanate, among other piezoelectric materials, has an unusually large electromechanical coupling coefficient K.sub.33 of 92%. However, this report describes only some dielectric characteristics, so the acoustic impedance, the dielectric loss, and the mechanical quality coefficient of the crystal, all of which are necessary in designing an ultrasonic probe, are still unknown. In particular, the report does not at all refer to the characteristics of strip ultrasonic transmitting/receiving elements which are widely used in ultrasonic probes. Moreover, some ultrasonic probes including a plurality of ultrasonic transmitting/receiving elements each having a strip piezoelectric element manufactured by slicing from the solid-solution single crystal cannot provide high-resolution images because their signal levels are low. The present inventors examined ultrasonic transmitting/receiving elements which were incorporated into ultrasonic probes with a low sensitivity and had these strip piezoelectric elements. Consequently, the present inventors found that the value of the apparent electromechanical coupling coefficient decreased from its initial value, and this was the cause of the low sensitivity. This decrease in the apparent electromechanical coupling coefficient can be improved by performing polarization by applying a high electric field again, and this increases the sensitivity of an ultrasonic probe. However, re-polarization in the manufacture is unpreferred since it increases the number of manufacturing steps to result in cumbersome operations. This also leads to a high cost or of degradation of performance resulting from changes with time.